Nanowicks are dense mats of nanoscale fibers that are expected to enable the development of a variety of novel capillary pumps, filters, and fluidic control devices. Nanowicks make it possible obtain a variety of novel effects, including capillary pressures orders of magnitude greater than those afforded by microscale and conventional macroscale wicks. While wicking serves the key purpose of transporting fluid, the nanofiber geometry of a nanowick makes it possible to exploit additional effects — most notably, efficient nanoscale mixing, fluidic effects for logic or control, and ultrafiltration (in which mats of nanofibers act as biomolecular sieves).
A nanowick (see Figure 1) typically consists of carbon nanotubes grown normal to a substrate in a tailorable pattern. The liquid of interest is constrained to flow in the interstices between the fibers. (In practice, the liquid must include a surfactant because carbon nanotubes are hydrophobic.) By suitable control of the growth process, the interfiber distance and/or the fiber length can be made to range from nanometers to millimeters and to vary with position (in one or two dimensions) on the substrate. Similarly, the fiber diameter can be made to vary with position. The spatial variation in spacing and/or diameter can be chosen to obtain such effects as prescribed spatial variations in wicking speed or prescribed degrees of separation among different biomolecules.
The following are examples of potential applications and potential variations in designs of nanowicks:
- Somewhat analogously to strips of litmus paper, wicking chips could be made as disposable devices for rapid testing of liquids. To start a test, a drop of liquid would be placed on top of the array of nanofibers on a wicking chip (see Figure 2). After absorption of the drop and transport of the liquid by wicking, the liquid could be filtered and analyzed (for viscosity, for example) in a very simple manner, without need for any complicated pumping mechanism.
- A liquid could be made to flow continuously, as in a capillary-pumped loop. The liquid would enter a nanowick at one end, would flow through the mat of fibers by capillary action, and would be made to evaporate at the other end. The evaporation would sustain the pumping action in the same manner in which evaporation of water from leaves sustains capillary pumping in living plants.
- A nanowick could serve as both a filter and a pump: While a liquid was flowing through a nanowick, the fibers could trap particles and large molecules (for example, protein and deoxyribonucleic acid molecules).
- The pattern of nanofibers could be tailored to exploit a combination of diffusion and extensional flow to promote nanoscale mixing of two liquids.
- Nanowicks could be patterned to act as various fluidic logic devices, including ones that exert fluidic effects analogous to the electrical effects of transistors and diodes. Unlike macroscale and microscale fluidic devices, the nanowickbased fluidic devices could, conceivably, be designed and built without channels and could operate without mechanical pumps.
- It might be possible to construct nanowicks in which selective wicking could be controlled electrically.
- Although capillary forces would suffice to contain a liquid within a nanowick, without need to place the wick in a channel, the nanowick could be capped, if desired, to prevent evaporation.
- Nanowicks could be used to transport liquids through interstitial spaces into which tubes could not inserted.
This work was done by Flavio Noca, Michael Bronikowski, Elijah Sansom, Jijie Zhou, and Morteza Gharib of Caltech for NASA's Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Materials category.
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